This adds a new prototype `cmd/natc` which can be used
to expose a services/domains to the tailnet.
It requires the user to specify a set of IPv4 prefixes
from the CGNAT range. It advertises these as normal subnet
routes. It listens for DNS on the first IP of the first range
provided to it.
When it gets a DNS query it allocates an IP for that domain
from the v4 range. Subsequent connections to the assigned IP
are then tcp proxied to the domain.
It is marked as a WIP prototype and requires the use of the
`TAILSCALE_USE_WIP_CODE` env var.
Updates tailscale/corp#20503
Signed-off-by: Maisem Ali <maisem@tailscale.com>
